Traditional bleaching processes of pulps commonly employ alkaline hydrogen peroxide as a bleaching agent. To that effect, hydrogen peroxide and a water soluble alkaline compound, such as sodium hydroxide and/or water glass are introduced into the pulp in order to improve whiteness and/or brightness of a wood pulp, a recycled paper pulp or any other mechanical pulp, such as a pulp for papermaking processes. The use of chelating agents like EDTA to remove some of the metal ions from the pulp prior to adding peroxide allows the peroxide to be used more efficiently. Magnesium salts and sodium silicate may also be added to improve bleaching with alkaline peroxide.
WO 94/12725 A1 discloses the use of metal hydroxide-aluminium chloride mixtures as an alkaline agent in hydrogen peroxide bleaching of virgin wood pulps and recycled paper pulps.
More recently (for example as disclosed in Li et al., Pulp & Paper Canada 106:6 (2005), pages T125 to T129 “Using magnesium hydroxide (Mg(OH)2) as the alkali source in peroxide bleaching at Irving paper”) magnesium hydroxide Mg(OH)2 and magnesium oxide MgO have been employed to replace sodium hydroxide and water glass, and to reduce the need for chelating agents and sodium silicate. Since magnesium hydroxide and alkaline earth metal hydroxides in general have limited solubility in water, only reduced amounts are discharged as waste water, making it economically and environmentally attractive to use. However, magnesium hydroxide is not used in a widespread manner due to issues with formation of deposits in the pulp resulting from the reaction of Mg2+ ions with soluble and colloidal wood components, in particular magnesium fatty acid salts and resins. The presence of these deposits unfavourably affects paper machine runnability and paper mill productivity. These problems have prevented alkaline earth metal hydroxide assisted hydrogen peroxide bleaching to be employed more widely.
The problem of magnesium based fatty acid salts and resins in pulps has so far received little attention in the specialised literature. It is therefore an aim of the present invention to allow the widespread use of alkaline earth metal hydroxides or alkaline earth metal oxides in the bleaching of pulps.